1. Field of the Invention
This invention relates to an optical modulation device. This invention also relates to a method of driving an optical modulation device.
2. Description of the Prior Art
Semiconductor lasers are frequently used as light sources in optical communication systems. A typical way of modulating an optical signal is to control the injection current of a semiconductor laser in response to a modulating signal (an information signal). However, it is generally difficult to operate a semiconductor laser at higher than several GHz frequencies by such direct modulation because of characteristics thereof which relate to a carrier life time.
External electro-optic waveguide modulators using dielectric crystals can perform higher-frequency modulation. A typical electro-optic waveguide modulator includes a transmission line and an optical waveguide. The transmission line serves as a modulating electrode which enables a modulating signal to propagate on an electro-optical crystal. The optical waveguide extends near the transmission line. An electric field is induced around the modulating electrode in response to the modulating signal. The electro-optical effect causes the refractive indices of the optical waveguide to vary with the induced electric field. Accordingly, the phase of a light wave in the optical waveguide is varied in response to the modulating signal so that optical modulation is realized.
General electro-optical crystals such as an LiNbO.sub.3 crystal do not have large electro-optical coefficients. Thus, in an electro-optical modulator, it is important to effectively apply a modulating electric field to an optical waveguide to attain a good modulation efficiency.